1. Field of the Invention
This invention generally relates to optical lens and, more particularly, to a micro array of lens assemblies made from optical fiber and an associated fabrication process.
2. Description of the Related Art
FIG. 1 is a conventional microscope objective with condenser lens (prior art). In the visual inspection and an analysis of integrated circuit (IC), a microscope is typically employed, as the IC features can be quite small. Conventionally, a single lens is used at a localized position, with lighting applied to the backside of an IC or an IC embedded in a package.
The ability of a microscope objective to capture deviated light rays from a specimen is dependent upon both the numerical aperture (NA) and the refractive index (n) of the medium through which the light travels, as follows:(NA)=n(sin m);
where m is one-half the angular aperture of the objective.
Because sin m cannot be greater than 90 degrees, the maximum possible numerical aperture is determined by the refractive index of the immersion medium. Most microscope objectives use air as the medium through which light rays must pass between the sample and front lens of the objective. Objectives of this type are referred to as dry objectives because they are used without liquid imaging media. Air has a refractive index of 1.0003, very close to that of a vacuum and considerably lower than most liquids, including water (n=1.33), glycerin (n=1.470) and common microscope immersion oils (average n=1.515). The index of refraction of the silicon die is 3.42. Fiber optic material is also available made of silicon, with an index of refraction that perfectly matches an IC die. For near infrared fused silica, fused quartz, or BK7, glass can be used. In practice, the maximum numerical aperture of a dry objective system is limited to 0.95, and greater values can only be achieved using optics designed for immersion media.
Microscope objectives designed for use with immersion oil have a number of advantages over those that are used dry. Immersion objectives are typically of higher correction (either fluorite or apochromatic) and can have working numerical apertures up to 1.40 when used with immersion oil having the proper dispersion and viscosity. These objectives allow the substage condenser diaphragm to be opened to a greater degree, thus extending the illumination of the specimen and taking advantage of the increased numerical aperture.
A factor that is commonly overlooked when using oil immersion objectives of increased numerical aperture is limitations placed on the system by the substage condenser. In a situation where an oil objective of NA=1.40 is being used to image a specimen with a substage condenser of smaller numerical aperture (1.0 for example), the lower numerical aperture of the condenser overrides that of the objective and the total NA of the system is limited to 1.0, the numerical aperture of the condenser. One complicated solution to this problem is to also use an oil medium between condenser lens system and the specimen.
The substage condenser gathers light from the microscope light source and concentrates it into a cone of light that illuminates the specimen with uniform intensity over the entire viewfield. It is critical that the condenser light cone be properly adjusted to optimize the intensity and angle of light entering the objective front lens. Each time an objective is changed, a corresponding adjustment must be performed on the substage condenser to provide the proper light cone for the numerical aperture of the new objective.
FIGS. 12A and 12B depict a cylindrical lens (prior art). A cylindrical lens focuses light along a single axis, forming a line image from incident parallel beams.
FIG. 13 depicts a plano-convex lens (prior art). A plano-convex lens is useful in collimating a parallel beam from a point of light. Two plan-convex lenses, oriented with their convex sides facing each other, act as a relay lens, and can be used to relay an image.
FIG. 2 is a diagram of a simple two-lens Abbe condenser (prior art). Light from the microscope illumination source passes through the condenser aperture diaphragm, located at the base of the condenser, and is concentrated by internal lens elements, which then projects light through the specimen in parallel bundles from every azimuth. The size and numerical aperture of the light cone is determined by adjustment of the aperture diaphragm. Correct positioning of the condenser with relation to the cone of illumination and focus is critical to quantitative microscopy and optimum photomicrography.
While a high magnification lens may be desirable for small, narrowly defined specimens, it may not be desirable for larger fields of view. As a result, if a conventional high magnification microscope is used, an IC failure or combination of connected IC features may only be understood by finding and viewing multiple high-magnification images. Further, high magnification lens are fragile, easily damaged, expensive, not made for large area viewing at low magnifications.
Generally, there is a need to efficiently collect light through the back of an IC, and transfer the light to a sensor or camera with minimal loss. With the emergence of IC back side analysis, there is a need for lenses that are compatible with the index of refraction of silicon, which is much higher than glass and air, that also have a high collection efficiency. That is, the numerical aperture must be such that it can be placed in contact with the planar surface of the back of the silicon die.
It would be advantageous if a single, high magnification silicon immersion lens (SIL) could be replaced with a micro array of lens having a low magnification, for a larger field of view in semiconductor contact analysis.